Recent concerns about volatility and toxicity of hydrocarbon fuel and the resultant environmental damage has made it desirable to limit the content and composition of aromatic hydrocarbons in such fuels and benzene in particular. Limiting benzene poses some of the most severe problems for the operation of catalytic reformers that have been relied on to raise the octane of unleaded fuels. Reformers that were typically operated at high severity to produce high octane benzene and aromatic hydrocarbons must now be operated to eliminate benzene and reduce aromatics. There are a variety of ways in which the operation of the reforming section may be altered to reduce the reformate benzene concentration. Changing the cut point of the naphtha feed split between the reforming and isomerization zones from 180 to 200.degree. F. will remove benzene, cyclohexane and methylcyclopentane from the reformer feed. Benzene can alternately also be removed from the reformate product by splitting the reformate into a heavy fraction and a light fraction that contains the majority of the benzene. Practicing either method will put a large quantity of benzene into the feed to other processing units, particularly the isomerization zone. Therefore, it is still necessary to have an efficient and cost effective means for removing benzene from feeds.
Octane improvement is also obtained by catalytically isomerizing the C.sub.4 -C.sub.6 paraffins hydrocarbons to rearrange the structure of the paraffinic hydrocarbons into branch-chained paraffins. Increasing amounts of the benzene that were formerly blended with finished gasoline products to increase octane are present in the isomerization feeds. It is known from U.S. Pat. No. 5,003,118 to saturate benzene in such feedstocks by the use of separate saturation zone upstream of the isomerization zone and that heat evolved from the saturation reaction can be used to heat the feed to the isomerization zone. As the benzene concentration of the feed varies, saturation of high benzene feeds can generate high heat inputs that excessively raise temperatures. High temperatures in the isomerization zone shift the equilibrium away from the production of the desired isoparaffins. Large changes in the concentration of benzene that are now often encountered when isomerizing light hydrocarbons make stable operation of isomerization processes difficult. Depending on the source of feed and the particular refining application benzene concentration can change by 20% or more on a daily basis.
The saturation of benzene in the feedstream also produces cyclohexane. U.S. Pat. No. 4,783,575 teaches the opening of cyclo-paraffinic rings in conjunction with the isomerization of light hydrocarbons using a platinum alumina catalyst. The opening of the cyclo-paraffnic rings increases the available paraffins for isomerization to C.sub.6 isoparaffins and provides further octane upgrading. Consequently, processes are sought for the elimination of benzene and the decyclization of the resulting saturated cyclic hydrocarbons that can accommodate widely varying quantities of benzene in the feed.